1. Field of the Invention
The present invention relates to a technology of generating triangular waves, and more particularly, to a technology of generating two phased triangular waves with phases opposite to each other.
2. Description of Related Art
Up to now, a switching regulator is often used for a device incorporating a single high-current power supply, such as a television device or a personal computer. In recent years, along with a demand for a reduction in size, a prolonged battery life, or the like of a mobile device, the switching regulator is generally implemented in the mobile device, and functional blocks such as a CPU, a memory, and a display panel are supplied with power using a single chip.
However, power supply voltages or currents required by the functional blocks are different from each other. Accordingly, in a case of supplying power to those functional blocks by using a single chip, when control thereof is performed with a single triangular wave as a reference, there is a limit to an increase in efficiency and a reduction in size of peripheral parts. If triangular waves with phases opposite to each other are used for the control by the switching regulator with an output of a multi-power supply, a further increase in efficiency and down-sizing of parts can be achieved.
Japanese Unexamined Patent Application Publication No. 2006-50310 discloses a triangular wave generation circuit for generating two phased triangular waves with phases opposite to each other. FIG. 11 shows the triangular wave generation circuit disclosed in Japanese Unexamined Patent Application Publication No. 2006-50310, and corresponds to FIG. 4 of Japanese Unexamined Patent Application Publication No. 2006-50310.
As shown in FIG. 11, the triangular wave generation circuit includes: a triangular wave generation circuit 10A; a triangular wave generation circuit 10B; a midpoint potential fixing unit 20 for fixing a midpoint potential VN between output voltages of an A-wave and a B-wave of the triangular wave generation circuits 10A and 10B at a constant value (reference midpoint potential VRN); and a mode switching unit 30 for switching an output voltage generation mode (up-slope waveform generation mode or down-slope waveform generation mode) of each of the triangular wave generation circuits 10A and 10B.
In the configuration shown in FIG. 11, the triangular wave generation circuits 10A and 10B share a source constant-current circuit 14M and a sink constant-current circuit 16M in a switching manner. Specifically, the source constant-current circuit 14M is selectively connected to one of a first node NA and a second node NB through a changeover switch 18P. The sink constant-current circuit 16M is selectively connected to one of the first node NA and the second node NB through a changeover switch 18Q. The changeover switches 18P and 18Q operate in response to a switching control signal CS from a switching control circuit 36 of the mode switching unit 30. The source constant-current circuit 14M is configured as a PMOS transistor having a constant bias. The sink constant-current circuit 16M is configured as an NMOS transistor having a variable bias, and an output of a midpoint potential control circuit 24 provided to the midpoint potential fixing unit 20 is input as a bias to a control terminal of the sink constant-current circuit 16M.
The midpoint potential fixing unit 20 includes a midpoint potential detection circuit 22 and the midpoint potential control circuit 24. The midpoint potential detection circuit 22 includes resistors 26A and 26B having the same resistance value, and detects the midpoint potential VN between the A-wave and the B-wave. The midpoint potential control circuit 24 controls an output current of the sink constant-current circuit 16M so that the midpoint potential VN becomes equal to the reference midpoint potential VRN supplied from a reference voltage generation circuit 28.
The mode switching unit 30 includes two comparators 34A and 34B and the switching control circuit 36. The comparators 34A and 34B monitor output voltages of the triangular wave generation circuits 10A and 10B, respectively, and output the monitoring results to the switching control circuit 36. The switching control circuit 36 causes the changeover switches 18P and 18Q to be switched in response to the switching control signal CS when the output voltage of one of the triangular wave generation circuits 10A and 10B reaches a reference upper limit crest value. As a result, the output voltage generation mode of one of the triangular wave generation circuits 10A and 10B, which has an output voltage that reaches the crest value, is switched from an up-slope waveform mode to a down-slope waveform mode. Further, the output voltage generation mode of the other of the triangular wave generation circuits 10A and 10B is switched from the down-slope waveform mode to the up-slope waveform mode.
The triangular wave generation circuits each having the configuration shown in FIG. 11 are provided for outputting triangular waves of the A-wave and the B-wave with phases opposite to each other as shown in FIG. 12. The A-wave and the B-wave have the same frequency and the same crest value. The midpoint potential VN between the A-wave and the B-wave is fixed at the reference midpoint potential VRN. A high level shown in FIG. 12 corresponds to the reference upper limit crest value, and a low level shown in FIG. 12 corresponds to a reference lower limit crest value.
Incidentally, when the capacitors 12A and 12B which are included in the triangular wave generation circuits 10A and 10B, respectively, do not vary in capacitance, oscillation waveforms as shown in FIG. 12 can be obtained with the triangular wave generation circuit having the configuration shown in FIG. 11. On the other hand, the present invention has recognized that when the capacitors 12A and 12B vary in capacitance, there may arise a problem in that it is impossible to maintain the crest values of the A-wave and the B-wave at the same level.
When the capacitors 12A and 12B, which are included in the triangular wave generation circuits 10A and 10B, respectively, vary in capacitance, in order to maintain the crest values of the A-wave and the B-wave at the same level, it is necessary to supply a discharging current according to the variation of each of the capacitors 12A and 12B. Incidentally, since current sources for discharging the capacitors 12A and 12B each serve as the sink constant-current circuit 16M, there occurs a difference in discharge time constant between the A-wave and the B-wave due to a relative error between the capacitors 12A and 12B. As a result, even by controlling the sink constant-current circuit 16M, it is impossible to control the midpoint VN between the A-wave and the B-wave to be matched with the reference midpoint potential VRN. Therefore, there is a fear that the A-wave and the B-wave that have the same frequency and different crest values, for example, oscillation waveforms as shown in FIG. 13 may be generated.